Time-base correction of television signals in various forms has been known from the time of the earliest practical VTR in the 1950s. Time-base correction typically seeks to reduce short term and/or long term timing variations in a television signal, particularly one reproduced by a VTR. Time-base correction may also seek to synchronize the horizontal synchronizing (hereinafter "horizontal sync" or "H sync") pulses to an external reference.
The first electronic time-base correctors employed analog circuitry using variable analog delay lines for altering the timing of signals containing time-base errors. When digital memories became available, so-called "digital" time-base correctors came into use. Such "digital" time-base correctors were analog devices in which the variable delay lines were implemented by an analog-to-digital (hereinafter "A/D") converter feeding a digital memory, the output of which was converted back into the analog domain by a digital-to-analog (hereinafter "D/A") converter. Variable delay was achieved by modulating either the clock of the A/D input or the D/A output. An example of such an approach is set forth in my U.S. Pat. No. 4,675,724 in which time-base correction is achieved by modulating the sampling clock in the A/D converter.
Prior art "digital" time-base correctors of the type just described require either (a) access to the A/D or D/A converter, which may not be possible, or (b) subjecting an already digitized television signal to A/D and D/A conversion, which results in signal degradation and cannot be practically implemented in a single integrated circuit device.
Time-base correction takes on a somewhat different meaning in the case in which an analog television signal has been digitized by a stable sampling clock which is asynchronous with respect to the horizontal line frequency of the television signal. In that case, the digitized television signal samples (a sample representing picture information is referred to as a "pixel") are timed in accordance with the stable sampling clock, but the underlying television signal is not in phase with the sampling clock and may contain time base errors. Time base errors may cause the time period of a horizontal line to be shorter or longer than the time period which would result at the nominal (i.e., long-term) horizontal line frequency of the television signal. Thus, because the sampling-clock rate remains constant while the horizontal line time period may vary, some lines may contain more samples or fewer samples than the number of samples per line resulting when the short-term frequency of the television signal is at its nominal frequency.
The problem of suppressing time-base errors in such digitized television signals is exacerbated (a) if access to the A/D converter is not available or (b) if such access is possible, if it is not desirable to digitize the analog television signal other than with a sampling clock which is asynchronous with respect to the horizontal line frequency of the television signal.
An example of the latter case is in the decoding of the analog signal from a VHS-type consumer VTR. There is no stable relationship between the color burst phase and the horizontal sync pulses of the signal. It is preferred to decode such signals into a component color signal having separated luminance and chrominance using an A/D converter with a clock synchronized to the signal's color burst rather than with a clock synchronized to the signal's horizontal sync (such a clock thus being asynchronous with respect to the horizontal sync). This is because burst locked decoding results in good luminance/chrominance separation (comb filters employed for such separation rely on the phase reversal of the chroma in alternate lines of an NETS signal, requiring precise phasing of the chroma with respect to the clock), whereas horizontal line locked decoding results in poor luminance/chrominance separation.
One prior art arrangement resamples a television signal already digitized by a clock asynchronous with the horizontal line frequency of the signal by producing new interpolated samples at times when a line-locked clock would have produced samples of the television signal. However, the interpolation is controlled by a feed-forward (open loop) control arrangement. Such an approach estimates future time-base error and seeks to correct it by measuring the time base error at the beginning of a horizontal scan line and applying the correction over the next scan line. Because it is a feed-forward control, the error measurement and correction applied as a result of the measurement must be very precise. Such an open loop system is not inherently self-correcting and may result in the accumulation of uncorrected errors. In addition, a feed-forward arrangement is likely to react too quickly to noise in the received signal.
Most time-base disturbances are caused by mechanical effects in the VTR: the scanner may speed up or slow down due to bearings or friction. The principal large time base error is at the beginning of the video field when the head comes "off" the tape and goes "on" again, making a full traverse from one edge of the tape to the other. If the tape is stretched too tightly or not tightly enough, there may be a tension error that can be half a line of video, hundreds and hundreds of pixels (a typical line may have in the order of 500 to 1200 pixels, depending on the clock frequency). Time-base correctors need not instantly correct for such large errors because such errors occur during the initial portion of each television field (up to about 20 lines) prior to the start of picture information and, thus, are not displayed by the reproducer (however, the presence of copy-protection signals in the vertical interval may shorten the number of lines available for recovery off the screen unless the time-base corrector removes the copy-protection signals). Consequently, it is generally sufficient for a time-base corrector to correct for such large time-base errors within 5 to 10 lines.
Smaller time base errors occur during the field when the video head is "on" the tape-typically a fraction of a sample up to a few samples. The magnitude of such large and small time-base errors depends on the quality of the VTR, consumer VTRs having time-base errors of a magnitude significantly higher than professional broadcast-quality VTRs.
A typical consumer television set is generally able to satisfactorily reproduce signals from consumer VTRs having time-base errors as just described. This is because such television sets have horizontal oscillators with time constants of about 5 to 10 lines, which provide a type of "built-in" time-base corrector action. However, unlike a conventional consumer television set, a monitor-type reproducer (such as a television projection monitor or a computer monitor) or digital signal processor (such as a line doubler or a digital-bit-rate reducer, e.g. video "compressor") requires a highly stable relationship between the pixel clock rate of the digital television signal and its synchronizing pulses.